High solids primer composition
BACKGROUND OF THE INVENTION 
     This invention is directed to a primer composition and in particular to a high solids primer having a low VOC content (volatile organic content) primarily useful for automobiles and trucks. 
     Primer compositions are well known in the art and are widely used in the manufacture of automobiles and trucks as shown in Simon U.S. Pat. No. 4,232,090 issued Nov. 4, 1980. High solids polyester coating compositions also are well known as shown in Chang U.S. Pat. No. 3,994,851 issued Nov. 30, 1976. Polyester copolymer coating compositions are shown in Piccirilli et al U.S. Pat. 4,205,115 issued May 27, 1980 and in Ambrose et al U.S. Pat. No. 4,535,132 issued Aug. 13, 1985. However, none of these compositions meet the current needs of modern automotive and truck manufacturing plants which require high solids compositions that have a low VOC but have excellent physical properties such as corrosion resistance and a high level of adhesion to primed and unprimed metal substrates, primed and unprimed plastic substrastes and provide a surface to which conventional topcoats will adhere. 
     SUMMARY OF THE INVENTION 
     A primer composition containing solvents and about 50-85% by weight of a film forming binder and pigments in a pigment to binder weight ratio of about 1:100-150:100; and the binder contains about 
     (1) 30-65% by weight, based on the weight of the binder, of a hydroxyl containing polyester copolymer having a hydroxyl number of about 100-300 and a number average molecular weight of about 300-3,000 and being the esterification product of an alkylene glycol, a branched chain polyol, a cycloaliphatic dicarboxylic acid, an aliphatic dicarboxylic acid and an aromatic dicarboxylic acid or anhydride thereof; 
     (2) 5-20% by weight, based on the weight of the binder, of a reactive plasticizer; and 
     (3) 30-50% by weight, based on the weight of the binder, of a monomeric fully alkylated melamine formaldehyde crosslinking agent. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The primer composition has a low VOC content (volatile organic content), can be formulated into a light colored composition, forms finishes that are hard, have excellent adhesion to a variety of substrates such as cold rolled steel, phosphatized steel, phosphatized steel primed with a primer applied by electrocoating, plastic substrates which may be primed or unprimed such as polyester reinforced fiber glass, reaction injection molded urethanes, partially crystalline polyamides and other plastic substrates and provides a surface to which conventional topcoats will adhere. 
     The primer composition is particularly useful on the aforementioned substrates since it can be used as a surfacer to cover imperfections in surfaces of primed metal and plastic substrates. For example, electrocoating of metal substrates with a primer often results in a finish that has small imperfections and this composition can be applied to form a smooth, glossy finish that is free from imperfections. Also, plastic substrates such as SCM (sheet molding compound) which is a polyester reinforced with fiber glass contains many surface imperfections and must be coated with a surfacer. By increasing the pigmentation used in the composition, an easily sandable finish is formed that covers imperfections and can be sanded to a smooth finish which is then topcoated with conventional acylic enamel finishes. 
     A corrosion resistant primer for metal substrates can be formed by replacing about 5-15% by weight of the polyester of the binder with an epoxy resin such as &#34;Epon&#34; 828 or 1001. 
     The primer composition has a film forming binder content of about 50-85% by weight and correspondingly about 15-50% by weight of a liquid carrier which usually is a solvent for the binder. 
     The film forming binder of the composition contains about 30-65% by weight of a polyester copolymer, about 5-20% by weight of castor oil and about 30-50% by weight of the monomeric fully alkylated melamine formaldehyde crosslinking agent. One preferred composition contains about 35-45% by weight of the polyester copolymer, 5-20% by weight of castor oil and 40-50% by weight of the crosslinking agent are used and forms a high quality composition. 
     The hydroxyl containing polyester copolymer used in the composition has a hydroxyl number of about 100-300 and a Mn (number average molecular weight) of about 300-3,000 and is the esterification product of an alkylene glycol, a branched polyol, a cycloaliphatic dicarboxylic acid, an aliphatic dicarboxylic acid and an aromatic dicarboxylic acid or anhydride thereof. 
     Molecular weight is determined by gel permeation chromatography using polymethyl methacrylate as the standard. 
     Typical alkylene glycols or diols that are useful for forming the polyester have from 2-10 carbon atoms and are for example ethylene glycol, propylene glycol, butane diol, pentane diol, neopentyl glycol, hexane diol, octane diol, Esterdiol 204 (a commercial diol produced by Union Carbide) and the like. One preferred gylcol is neopentyl glycol since it forms a high quality polyester. 
     Typical branched chain polyols that can be used are trimethylolpropane, triethylolpropane, pentaerythritol and the like. One preferred branched chain polyol is trimethylolpropane. 
     Typical cycloaliphatic dicarboxylic acids that can be used are alkyl cyclohexyl dicarboxylate such as dimethyl cyclohexyl dicarboxylate. 
     Typical aliphatic dicarboxylic acids that can be used are adipic acid, azelaic acid, succinic acid, glutaric acid, pimelic acid, suberic acid, and the like. 
     Typical aromatic dicarboxylic acids that can be used are phthalic acid, orthophthalic acid, isophthalic acid and the like. 
     Typical anhydrides that can be used are orthophthalic anhydride, succinic anhydride, glutaric anhydride and the like. 
     One preferred polyester copolymer is the esterification product of neopentyl gylcol, trimethylolpropane, dimethyl cyclohexyl dicarboxylate, adipic acid and isophthalic acid having an acid no. of about 5-20, a hydroxyl no. of about 150-200 and a Mn of about 1000-2200. 
     The polyester copolymer is prepared by conventional esterification procedures in which the components are charged into a polymerization vessel with solvents and usually a esterification catalyst and heated to about 100°-200° C. for about 30 minutes to 5 hours with removal of water that is formed during the process. 
     Castor oil is used as a reactive plasticizer. Other reactive plasticizers can be used such as polycaprolactone. 
     The crosslinking agent used in the composition is a monomeric fully alkylated melamine formaldehyde crosslinking agent. Preferably, the crosslinking agent is a monomeric butylated and methylated melamine formaldehyde, butylated and methylated in a 1:1 molar ratio. One commercially available agent of this type is &#34;Resimine&#34; 755. 
     Typical pigments that can be used in the composition are filler pigments such as talc, china clay, barytes, carbonates, silicates, metallic oxides such as titanium dioxide, zinc oxide and iron oxide and carbon black and organic colored pigments and dyes. The resulting primer composition has a pigment to binder weight ratio of about 1:100-150:100. 
     The pigments can be introduced into the primer composition by first forming a mill base with the polyester copolymer or with another compatible polymer or dispersant by conventional techniques such as sand grinding, ball milling or attritor grinding. The mill base is blended with other constituents used in the composition. 
     Any of the conventional solvents or blends of solvents can be used to form the primer composition provided that the selection of solvents is such that the polymeric binder constituents are compatible and give a high quality primer. The following are examples of solvents that can be used to prepare the composition: methyl ethyl ketone, methyl amyl ketone, methyl isobutyl ketone, toluene, xylene, acetone, ethylene glycol monobutyl ether acetate and other esters, ethers, ketones and aliphatic and aromatic hydrocarbon solvents that are conventionally used. 
     In one particularly preferred primer composition, the binder contains about 35-45% by weight, based on the weight of the binder, of a hydroxyl containing polyester copolymer having a hydroxyl number of about 100-300 and a number average molecular weight of about 300-3,000 and being the esterification product of neopentyl glycol, trimethylolpropane, dimethyl cyclohexyl dicarboxylate, adipic acid and isophthalic acid; 5-20% by weight, based on the weight of the binder, of castor oil, and 40-50 % by weight, based on the weight of the binder of a monomeric fully alkylated melamine formaldehyde crosslinking agent which is methylated and butylated in a 1:1 molar ratio. The composition forms a hard cured coating on a substrate having excellent adhesion to the substrate and forms a smooth finish to which conventional topcoats can be applied. 
     The composition can contain about 0.01-2% by weight, based on the weight of the binder, of ultraviolet light stabilizers which term includes ultraviolet light absorbers, screeners and quenchers. Typical ultraviolet light stablizers include benzophenones, triazines, triazols, benzoates, hindered amines and blends of thereof. 
     The primer composition can be applied to a plastic or metal substrate by conventional techniques such as spraying, electrostatic spraying, dipping, brushing, flowcoating and the like. As mentioned above the preferred method is electrostatic spraying. After application, the composition is baked at about 120°-200° C. for about 5-45 minutes to form a primer coating layer about 0.1-2.0 mils thick. Generally the primer layer is about 0.5-1.5 mils thick. 
     To achieve an adequate cure of the composition, a blocked acid catalyst is added to the composition in amounts of about 0.1-6.0% by weight of the binder. Typically useful blocked acid catalysts are aromatic sulfonic acids blocked with amino methyl propanol or dimethyl oxazoline. Typically useful aromatic sulfonic acids are paral toluene sulfonic acid, dodecyl benzene sulfonic acid, decyl benzene sulfonic acid. One preferred catalyst is dodecyl benzene sulfonic acid blocked with amino methyl propanol. 
     Conventional solvent based or water based acrylic enamels or lacquers, acrylic polyurethane coatings, polyesterurethane coatings, alkyd enamels and the like can be applied by electrostatic spraying over the primer and then baked to form a durable automotive or truck finish. 
     The following example illustrates the invention. All parts and percentages are on a weight basis unless otherwise indicated. Molecular weights are determined by gel permeation chromatography using polymethyl methacrylate as the standard. 
     EXAMPLE 1 
     A primer composition is prepared by first forming a mill base and then mixing the mill base with the components of the primer composition. 
     
         ______________________________________                                    
                           Parts                                          
Gray Mill Base             By Weight                                      
______________________________________                                    
Portion 1                                                                 
Polyester resin solution (80% resins solids in                            
                           20.00                                          
methyl amyl ketone of a polyester resin of                                
neopentyl glycol, trimethylolpropane, dimethyl                            
cyclohexyl dicarboxylate, adipic acid and                                 
isophthalic acid in a weight ratio of                                     
37.9/8.6/22.8/5.5/25.2 having an acid number of                           
about 10-15 and a hydroxyl no. of 170-190 and                             
a Gardner Holdt visc. of Z-Z.sub.2 and an Mn of 1500)                     
Castor Oil                 2.00                                           
&#34;Resimine&#34; 755 (fully butlylated and methylated                           
                           9.00                                           
in a 1:1 molar ratio monomeric melamine                                   
formalehyde crosslinking agent)                                           
Methyl amyl ketone         20.55                                          
Portion 2                                                                 
Titanium dioxide pigment   45.74                                          
Carbon black pigment (Vulcan XC-72-R Furnace                              
                           2.71                                           
Black pigment without a surface treatment)                                
Total                      100.00                                         
______________________________________                                    
 
    
     The above constituents are charged into a conventional sand mill and ground to a fineness of 0.5 mils. 
     A primer was prepared by blending together the following constituents: 
     
         ______________________________________                                    
                      Parts                                               
                      by Weight                                           
______________________________________                                    
Gray Mill Base (prepared above)                                           
                        743.82                                            
Polyester resin solution (described                                       
                        91.91                                             
above)                                                                    
Castor oil              57.26                                             
&#34;Resimine&#34; 755 (described above)                                          
                        149.66                                            
Flow and leveling agent (1% Silicone oil                                  
                        2.90                                              
solution in butyl acetate)                                                
Blocked acid catalyst solution                                            
                        14.44                                             
(53% solids dodecylbenzene sulfonic acid                                  
blocked with dimethyloxazolidine in                                       
methanol)                                                                 
Methyl amyl ketone      27.62                                             
Total                   1087.61                                           
______________________________________                                    
 
    
     The resulting primer composition has a solids content of 78%, a pigment/binder ratio of 42/100 and is gray in color. 
     The primer was reduced to a spray viscosity of 35&#34;measured with a No. 2 Fisher cup at 25° C. with methyl amyl ketone and sprayed onto a cold roll steel panel, a phosphatized steel panel, a fiber glass reinforced polyester panel, a reaction injection molded urethane panel and a partially crystalline polyamide panel. The steel panels were baked for 30 minutes at about 125° C. and the remaining panels were baked at 120° C. for about 30 minutes. The panels had had a gray color, a dry film thickness of about 0.8 mils and a gloss measured at 60° C. of about 30. 
     The coatings on the panels had the following properties: 
     
         ______________________________________                                    
mar resistance - good                                                     
adhesion - good                                                           
sandability - good                                                        
knife scrape resistance - good                                            
solvent resistance - &gt;50 double rubs                                      
(1:1 ratio p-naphtha/isopropanol)                                         
humidity resistance                                                       
96 hour at 38° C./100% rel. humidity - 10.sup.1                    
dry crosshatch and tape adhesion - 10.sup.1                               
chip resistance - gravelometer - 9.sup.1                                  
______________________________________                                    
 Rating system                                                            
 .sup.1 0-10, 10  best, 0  worst                                          
 .sup.2 0-5, 5  best, 0  worst                                            
 
    
     EXAMPLE 2 
     A gray surfacer was prepared by forming a gray mill base and then formulating the surfacer. 
     
         ______________________________________                                    
                        Parts                                             
Gray Mill Base (32-8944)                                                  
                        By Weight                                         
______________________________________                                    
Portion 1                                                                 
Polyester resin solution (described in                                    
                        20.00                                             
Example 1)                                                                
&#34;Resimine&#34; 755 (described in Example 1)                                   
                        5.00                                              
Butyl acetate           8.00                                              
Xylene                  2.00                                              
Portion 2                                                                 
Titanium dioxide pigment                                                  
                        64.86                                             
Carbon black pigment (described in                                        
                        0.14                                              
Example 1)                                                                
Total                   100.00                                            
______________________________________                                    
 
    
     The above constitutents are charged into a conventional sand mill and ground to a fineness of 0.5 mils. 
     
         ______________________________________                                    
                        Parts                                             
(764-171)               by Weight                                         
______________________________________                                    
Portion 1                                                                 
Gray Mill Base (prepared above)                                           
                        370.45                                            
Silicone oil solution (1% solution)                                       
                        3.61                                              
Blocked acid catalyst solution                                            
                        4.31                                              
(53% solids dodecylbenzene sulfonic acid                                  
blocked with dimethyloxazolidine in                                       
methanol)                                                                 
Portion 2                                                                 
Polyester resin solution (described                                       
                        226.81                                            
above)                                                                    
Castor oil              90.26                                             
&#34;Resimine&#34; 755 (described in Example 1)                                   
                        237.82                                            
Fumed Silica dispersion (8.9% silica in a                                 
                        29.01                                             
melamine resin solution)                                                  
Portion 3                                                                 
Butanol                 36.11                                             
Polyvinyl pyrrolidone anti-sag agent                                      
                        5.42                                              
(3% in butynol)                                                           
Portion 4                                                                 
Xylene                  12.04                                             
Methyl ethyl ketone     21.66                                             
Total                   1037.50                                           
______________________________________                                    
 
    
     The resulting surfacer has a solids content of 82%, a pigment/binder ratio of 42/100 and is light gray in color. 
     The primer was reduced to a spray viscosity of 35&#34; measured with a No.2 Fisher cup at 25° C. with methyl ethyl ketone and sprayed onto a cold roll steel panel, a primed and phosphatized steel panel, a fiber glass reinforced polyester panel, a reaction injection molded urethane panel and a partially crystalline polyamide panel. The steel panels were baked for 30 minutes at about 150° C. and the remaining panels were baked at 120° C. for about 30 minutes. The acid catalyst level was increased by a factor of 3 at the 120° C. bake. The panels had had a gray color, a dry film thickness of about 0.8 mils and a gloss measured at 60° C. of about 50. 
     The coatings on the panels had the following properties: 
     
         ______________________________________                                    
adhesion - good                                                           
sandability - good                                                        
knife scrape resistance - good                                            
solvent resistance - &gt;50 double rubs                                      
(1:1 ratio p-naphtha/isopropanol)                                         
humidity resistance                                                       
96 hour at 38° C./100% rel. humidity - 10.sup.1                    
dry crosshatch and tape adhesion - 10.sup.1                               
chip resistance - gravelometer - 10.sup.1                                 
______________________________________                                    
 Rating system                                                            
 .sup.1 0-10, 10  best, 0  worst                                          
 .sup.2 0-5, 5  best, 0  worst